Ultrasound technology is utilized for measuring fluid/gas velocity through a pipe of known dimensions, with ultrasound transducers placed within the pipes to determine the fluid/gas velocity and the fluid/gas volume flow, which is measured by multiplying fluid/gas velocity by the interior area of the pipe. Cumulative fluid volume may be measured by integrating fluid flow over time. To provide ultrasonic flow metering, two transducers are spaced apart within a pipe such that a first signal is sent in the upstream direction and a second signal is sent in the downstream direction. The time-of-flight (TOF) for both signals is measured, with the difference between the two measurements indicating the amount of flow within the pipe. Reciprocal operation in the two directions is critical as the measurements must attain a high degree of accuracy, e.g., in order to detect small leaks. A tuned flow meter can detect a difference in TOF in the range of picoseconds. Providing the necessary reciprocal operation means attaining zero differential TOF at zero flow despite mismatches in transducer pairs. Achieving this goal requires perfect matching of electrical impedances between the transmitting circuit and the receiving circuit. A common way to attain matching is by designing the ultrasonic hardware to be voltage transmit, i.e., with close to zero impedance on the driver side, and current receive, i.e., with close to zero impedance on the receiver side. Attaining zero impedance places stringent requirements on both the driver and the receiver circuits to behave as ideal circuits.
Several factors further complicate this process:                Although it is possible to correct a known offset of the signals during processing of the signals, any offset present between the two circuits will vary with the ambient and propagation medium temperature. Thus, in order to apply such corrections during processing, a temperature gauge is necessary for accurate flow measurements. The standards for flow meters require testing of the meter at zero flow over a range of temperatures.        The standards further require that flow meters be designed for a life span of 15-20 years. Over time, changes can occur, either within the pipe or to the transducer itself, e.g., sedimentation, corrosion, aging, etc., causing drifting of the acoustical impedance. When this occurs, it may be necessary to either retune or replace the flow meter, both of which require attention from a technician.        Upstream and downstream signals need to match point for point for perfect reciprocity, but when flow is occurring within the pipe, it is both tedious and difficult to perform this type of matching. Stopping the flow is often undesirable or impossible in the field.        